1. Technical Field
This invention relates to re-imaging optical systems, and more particularly to a re-imaging optical system incorporating a combination of refractive and diffractive optical elements for minimizing aberrations.
2. Discussion
The need to correct optical aberrations is pervasive in a wide variety of optical systems. Refractive and reflective components such as lenses, mirrors and prisms unavoidably introduce numerous kinds of optical aberrations. These include, for example, lateral and axial chromatic aberrations, coma, spherical chromatism, chromatic coma, astigmatism, field (Petzval) curvature, etc.
The more complex an optical system is, the more refractive elements the system requires. This results in an increase in the aberrations induced by these refractive elements. As a result, additional refractive elements are required to correct these aberrations.
For example, many optical systems employing optical sensors need to utilize a re-imaging optical configuration. A re-imaging optical configuration in an optical sensor system offers a number of advantages such as better off-axis radiation rejection, 100 percent cold shielding with the cold stop next to the detector module and an accessible entrance pupil. In general, a re-imaging optical system consists of two modules: an objective group for focusing an incoming beam of light onto an intermediate image plane, and a relay group following the intermediate image. The relay group then transmits the light to the detector. In a focal type re-imaging system, the relay group refocuses the light in a second image plane at the detector. In an afocal system, on the other hand, the relay group will collimate the light for viewing such as is commonly done in telescope systems.
Due to the added optical components in a reimaging optical system, a number of optical aberrations are introduced. For example, to obtain good image quality in a re-imaging optical system, the axial color of both the objective and relay groups have to be individually well corrected. While in principle, the objective group axial color can be removed by the relay group, the required additional negative power in the rear group will upset the third order aberration balances, and in addition will generate additional higher order aberrations. Further, spherical aberration and coma of both the objective and relay groups must be partially corrected within each group. Otherwise, the "stop" shift (non-zero chief height) induced astigmatism cannot be balanced out.
Because of these factors, refractive re-imaging optical systems are generally complicated and require many optical elements. In some cases, lenses with aspheric surfaces are employed. This results in a number of disadvantages. For example, the extra elements add to the cost of the system. In particular, aspheric surfaces which are used to prevent some types of aberrations, are relatively expensive. Further, these extra optical elements add considerably to the bulk and weight of the overall system. In addition, the additional optical elements affect image quality and result in losses in transmission.
For these reasons, it would be desirable to provide a re-imaging optical system which reduces the overall number of optical elements required. To this end, it would be desirable to have optical elements which do not introduce significant aberrations, and which can correct aberrations with a minimum number of components. Further it would be desirable to have a re-imaging optical system which is relatively inexpensive and which has good performance providing high image quality and low transmission losses. In addition it would be desirable to have a re-imaging optical system which is light and can be packaged in a relatively compact space.